This invention relates to a flying spot raster scanner, and, more particularly, to a rotatable aperture for a flying spot raster scanner optical system which adjusts the resolution along the scan line in the scan and cross-scan axes by adjusting the size of the imaged spot or pixel along the scan line.
Flying spot raster scanners contained in the prior art have a light source, such as a laser, which emits a coherent light beam. The light is collimated in both the scan or tangential axis and in the cross-scan or sagittal axis by multiple optical elements. The collimated light is focused in the cross-scan plane by a cylindrical optical element at a point near a facet of a rotating polygon mirror while remaining collimated in the scan plane.
The light is reflected from the facet and this reflected light revolves about an axis near the center of rotation of the rotating polygon mirror. The reflected, rotating light is focussed in the scan plane by spherical lenses and a cylindrical wobble correction mirror to scan along a line. This reflected light can be utilized to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photosensitive medium, such as a xerographic drum, in the output mode as a raster output scanner.
The imaged spots or pixels extend along the scan line. Adjusting the pixels in the scan axis will change the width of the pixels and thus altering the number of spots per inch resolution along the scan line. Adjusting the imaged pixels in the cross-scan axis will change the height of the pixels and thus alter the number of scan lines per inch along the perpendicular to the scan line.
The number of pixels per inch along a scan line and the number of scans per inch are set by the optical scanning system parameters.
The spot sizes at the scan line are fixed in the cross-scan or sagittal axis but can be varied in the scan or tangential axis. If the raster output scanner optical system is required to change resolutions in the cross-scan axis, it is most commonly done by switching a new cylindrical optical element into the beam's path before the polygon mirror while withdrawing the previous cross-scan focussing cylindrical optical element. This optical element replacement requires a complicated and precision mechanical assembly to accomplish this task.
Raster scanners, both input and output, currently available on the market offer a single resolution for printing. The most common resolutions are 240, 300, 400,480 and 600 spots per inch (spi). The machine resolution is driven not only by the market need of the scanners, but by the required performance for the scanners, by the cost of the scanners and also by corporate strategy. For example, IBM Corporation uses a 240 and 480 spi resolution standard while Xerox Corporation uses a 300 and 400 spi resolution standard. Therefore, interchangeability between scanners with different printing resolutions becomes a problem.
Historically, the interchangeability problem was solved by using software interpolation programs to convert from one resolution to another. However, these software programs allow adjustable resolution in the scan axis only. It is desirable from a cost and customer satisfaction point of view to be able to adjust the resolution in both the scan and cross- scan axes. As performance requirements for the raster scanner change causing the number of scans per inch to change, the sagittal optical spot size or scan lines per inch must be adjusted. In this fashion, only limited software and hardware would be needed to interconnect raster output scanners with different resolutions.
It is an object of this invention to provide mechanical means to adjust the resolution of a flying spot raster scanner along the scan line in both the scan and cross- scan axes.